Bhama and Stupp in Polymer Engineering and Science, volume 30, number 4, pages 228 to 233 (February, 1990) disclose that carbon fibers dispersed into a thermotropic liquid crystal polyester have an influence on the orientation Of said polyester when a magnetic field is applied to induce orientation. An enhanced rate of magnetic field induced orientation wa3 observed in the molten polyester when carbon fibers were dispersed therein. Bhama and Stupp in Polymer Science and Engineering, volume 30, number 10, pages 603 to 608 (May, 1990) additionally disclose that carbon fibers which have been grafted with thermotropic liquid crystal polyester may be useful to control bonding and molecular orientation in the interfacial regions of the composite.
Buckley, et al. in U.S. Pat. No. 4,479,999 (Oct. 30, 1984) discloses a fabric comprising fusible and infusible fibers wherein said fusible fibers comprise thermotropic liquid crystal polymers. Heating of the liquid crystal fibers induces fusion to the adjacent infusible fibers without substantial loss of the orientation which was imparted to said fibers via melt extrusion.
Chung, Gurion and Stamatoff in Polymer Composites, volume 6, number 3, pages 181 to 184 (July, 1986) disclose that preoriented thermotropic liquid crystal copolyester films are selectively oriented parallel to the carbon fiber axis in compression molded composites prepared using continous carbon fiber. The parallel orientation occurs regardless of the initial direction of preorientation of the liquid crystal copolyester film.
All of the aforementioned employ a thermotropic liquid crystal thermoplastic matrix polymer to form the composite. Because of the extensive use of thermoset resins as composite matrix materials, it would be highly desireable to provide a process wherein a mesogenic thermoset resin matrix is substantially oriented during forming of a composite. Said composites would then provide needed increases in physical, mechanical and/or thermal properties as well as corrosion resistance over conventional thermoset resin based composites.
Current methods for achieving an oriented liquid crystal matrix in a composite system are based on either A.) injection molding or extrusion of blends of thermotropic liquid crystal thermoplastics and chopped fiber reinforcement or B.) consolidation of oriented films of thermotropic liquid crystal thermoplastics and a continuous fiber reinforcement. In the injection molding or extrusion processing methods, the amount of chopped fiber reinforcement is usually limited to 30 weight percent or less in order to maintain processability. This constraint greatly restricts the reinforcement that can be achieved. In the consolidation of oriented films of thermotropic liquid crystal thermoplastics, fiber wetout cannot be achieved without melting of the film. Melting and flow to wetout the fibers can result in partial, if not total loss of the orientation.
The present invention provides a process for orienting a mesogenic epoxy resin matrix during the preparation of a continuous fiber reinforced composite. According to the process of the present invention thermosettable mixtures containing a mesogenic thermoset resin are used to prepare composites wherein said cured resin provides an oriented matrix. Additionally, according to the process of the present invention, high levels of fiber reinforcement (50 weight percent or more) can be used and fiber wetout is achieved prior to and/or during orientation of the matrix.